Because links are hard.

LimeChat: http://limechat.net/mac/

Textual: http://www.codeux.com/textual/

Colloquy: http://colloquy.info/

WeeChat: http://weechat.org/

IRCCloud: https://irccloud.com (web-based, invite/beta)

I've been using Colloquy, but I'm open to trying other apps.